The research proposed will seek to define characteristics of the uterine epithelial plasma membrane and its surface coat as related to development of receptivity for ovo-implantation in the rabbit. Methods will include use of light and electron microscopic probes such as conjugated lectins, cationic ferritin, alkaline phosphatase cytochemistry, and freeze-fracture replicas for studies on membrane structure. Selected biochemical procedures will be employed to determine possible changes in the pattern of surface membrane glycoproteins related to uterine receptivity. The role of estradiol and progesterone in regulating these membrane surface properties will be assessed in vivo as well as on cultured uterine epithelial cells.